Non-contact position sensing finds is used in many applications, such as determining the position of an elevator in an elevator shaft, encoding the position of a stylus on a graphic tablet, and determining the positions of pieces on a game board.
Examples of systems that detect the position of an object and encode its position are disclosed in U.S. Pat. No. 5,815,091, entitled “POSITION ENCODER” issued to Dames et al. on Sep. 29, 1998, which is hereby incorporated by reference, U.S. Pat. No. 4,005,396, entitled “DIGITAL DISPLACEMENT SENSOR WITH ZIGZAG COILS” issued to Fujiwara et al. on Jan. 25, 1977, which is hereby incorporated by reference, and U.S. Pat. No. 6,124,708, entitled “POSITION DETECTION USING A SPACED APART ARRAY OF MAGNETIC FIELD GENERATORS AND PLURAL SENSING LOOP CIRCUITS OFFSET FROM ONE ANOTHER IN THE MEASUREMENT DIRECTION” issued to Dames on Sep. 26, 2000, which is hereby incorporated by reference.
Fujiwara describes a system using a movable element having a coil connected to an AC (alternating current) source. A stationary element, such as a tablet, has a number of windings mounted on its surface, and the windings have different periods. The position of the movable element relative to the stationary element is determined by the variation in mutual inductance between the coil and the windings. When the coil is energized, the magnitude of the current induced in a winding depends on the position of the coil relative to the winding, producing a signal that can be used by the system to determine the position of the movable element. Other systems may use different configurations of windings, but are similar in that the movable element has a coil that is energized to induce the signal in the windings of the stationary element.
Dames discloses a system comprising a support upon which a pair of phase quadrature windings is mounted. The windings are arranged to have a sinusoidal magnetic sensitivity characteristic along the length of the support. A movable element has a resonant circuit including a coil and capacitor that can magnetically couple with the windings. Excitation of the circuit induces currents in the windings that are dependent upon the position of the circuit within a period of the windings, and the system can determine the position of the movable element with respect to the support along an axis. A similar pair of windings is arranged on the support, orthogonal to the first pair of windings, to provide position detection along an axis orthogonal to the first. Thus, the system is able to detect the x-y position of the movable element with respect to the support.
However, such systems have shown limitations in the maximum size of the sensing area and the number of individual objects that can be detected simultaneously. This requires either limiting the size of the entire system or reducing the number of distinct movable objects in the system. It is also possible to construct a system using a series of adjacent sensing subsystems, but such a system would have uneven response at the junctions between subsystems, resulting in reduced accuracy and precision in detecting and encoding the positions of objects.
There exists a need, therefore, for an improved system and method for accurately detecting the position of objects in a large sensing area.